The invention relates to a semiconductor structure, particularly in a semiconductor detector, as well as to an associated operating method.
Semiconductor drift detectors for the detection of radiation have been known for some time and are described, e.g., in LUTZ, Gerhard: “Semiconductor Radiation Detectors”, Springer-Verlag (1999), STRÜDER, Lothar: “Nuclear Instruments and Methods in Physics Research A”, volume 454 (2000) and DE 34 27 476 A1, DE 102 13 812 A1 as well as DE 10 2004 004 283 A1. Here, the radiation to be detected generates signal electrons in a weakly doped, depleted semiconductor substrate and several ring-shaped and concentrically arranged electrodes are arranged on a surface of the semiconductor substrate that generate a drift field in the semiconductor substrate through which the signal electrons generated by the radiation drift to a centrally arranged readout element that detects the signal electrons and therewith the received radiation.
The readout element can consist here of a DEPFET transistor (DEPFET—Depleted Field Effect Transistor) which was invented in 1984 by J. KEMMER and G. LUTZ and is described, e.g., in DE 10 2004 004 283 A1.
Such a DEPFET transistor can comprise a weakly n-doped, depleted semiconductor substrate, a strongly p-doped back electrode being arranged on a surface of the semiconductor substrate that forms a diode poled in the reverse direction with the weakly n-doped semiconductor substrate and serving to deplete the semiconductor substrate, and holes produced in the semiconductor substrate by the action of radiation are removed by suction via the back electrode from the semiconductor substrate.
A strongly p-doped source region and a likewise strongly p-doped drain region are located on the opposite surface of the semiconductor substrate in a DEPFET transistor as readout element, and a conduction channel can develop between the source region and the drain region whose conductivity can be adjusted by an externally controllable gate electrode.
A weakly n-doped inner gate region is located below the conduction channel in the semiconductor substrate in which region the signal electrons produced in the semiconductor substrate by the action of radiation accumulate. The electrical charge accumulated in the inner gate region controls the conductivity of the conduction channel between the source region and the drain region in a manner similar to that of the external gate electrode so that the drain-source current is a measure for the detected radiation.
However, the signal electrons accumulated in the inner gate region must occasionally be removed from the inner gate region in order to preserve the sensitivity of the drift detector. To this end a separate clear contact is provided in the known DEPFET that is arranged, e.g., on the source side adjacent to the actual DEPFET transistor and removes by suction the signal electrons accumulated in the inner gate region by applying a positive electrical voltage.
In the previously described known semiconductor detector the drain-source current of the DEPFET transistor used as readout element is therefore a measure for the quantity of the signal charge that was produced by the radiation to be detected in the semiconductor substrate and which is accumulated in the inner gate region of the DEPFET transistor. However, in order to measure the radiation, the drain-source current need not be directly measured when the signal charge generated by the radiation to be detected arrives in the inner gate region of the DEPFET transistor, but rather it is sufficient for measuring the radiation if the drain-source current of the DEPFET transistor is measured at the beginning of a measuring time period and at the end of a measuring time period, wherein the DEPFET transistor can be switched off before, after and between the two measurements. Then, the radiation incident during the measuring time period follows from the difference of the two drain-source currents at the beginning and at the end of the measuring time period. It is appropriate here on account of the electronic noise to perform the two successive measurements of the drain-source current in the shortest possible time interval. However, this is difficult, e.g., in the case of a semiconductor detector with a plurality of picture elements since the individual picture elements of the semiconductor detector are read out sequentially in this case so that a given readout cycle time is present between two successive measurements of the drain-source current that depends on the number of the picture elements and the duration of the individual measuring procedures. This problem is posed, e.g., in applications in adaptive optics and on the so-called XFEL (X-Ray Free Electron Laser).
This problem can be solved in that in a readout procedure at first the drain-source current is measured with charge accumulated in the inner gate region, the inner gate region is subsequently completely emptied and the drain-source current is then measured again with the inner gate region being emptied. Finally, the difference of the two measured drain-source currents is then calculated as a measure for the radiation to be detected.
However, in this measuring method the first measurement not only reproduces the signal charge that was produced by the radiation to be detected in the semiconductor substrate and which is accumulated in the inner gate region, but rather the first measurement of the drain-source current also contains a disturbance signal that stems from the thermally generated charge (“dark current”) as well as from scattered light from other objects and which falsifies the radiation measurement.
Furthermore, radiation events are frequently observed whose exact point in time is known a priori. This is the case, e.g., in experiments with particle accelerators in which the particle beam has a regular, pulse-shaped time structure. It is desirable in these measurements to limit the sensitivity of the semiconductor detector to a given time window in which the radiation event of interest takes place. The time range between the individual radiation events can then be used to read out the measured signal charges.
The previously described known semiconductor detectors with a DEPFET transistor as a readout element therefore have the disadvantageous fact that the sensitivity of the semiconductor detector can not be limited in time relative to disturbance signals such as, e.g., thermally generated charge or scattered light from other objects without also clearing the signal charge carriers accumulated in the inner gate region that stem from the radiation of interest.
It is therefore an object of the invention to create a semiconductor detector that can be switched insensitive without clearing the signal charge carriers accumulated in the inner gate region.